3. Biomes - Lecture notes 3 PDF

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3. BIOMES 3.1. TROPICAL BIOMES a) Tropical rainforest Annual rainfall is greater than 2,000 mm (80 in.) and falls throughout the year Distinct layers of plants: -

canopy (‘ceiling’), consists of hundreds of species of broad-leaved, evergreen trees 30–40 m tall. Occasional emergent trees poke above the rest of the canopy, reaching 55 m. Little light filters through the thick canopy to the layers of vegetation in the mid-and understory. However, fallen trees create openings or gaps in the canopy; light that shines through these openings allows saplings, shrubs, and herbs to flourish.

Consistently favorable climate à net primary production in tropical rain forests is much greater than in any other terrestrial biome. Mainly due to rapid recycling of nutrients rather than on nutrients stored in soil. Hotspot: hosts more animal species than any other biome.

b) Tropical seasonal forest/ Dry forest Annual rainfall diminishes to 1,500–2,500 mm, and months with little or no rain become more common. Wettest regions of this biome support forests with canopy trees that are 40 m tall ≠ the driest regions support only scrubby woodlands à When the dry season begins, many trees lose their leaves, a response that reduces their need for water. Such trees are said to be drought deciduous In tropical seasonal forests, wet and dry seasons each span about half of the year.

c) Tropical savanna Rainfall is highly seasonal and drought conditions generally persist for more than half of the year. Open grasslands and a few scattered trees à Like the trees of the tropical seasonal forest, many of the trees that live in the tropical savanna are drought deciduous. Tropical savanna is a biome of seasonal feast and famine à With the rain, grasslands flourish, supporting large populations of herbivore animals, which feed a diverse array predator, including lions and cheetahs. ≠ When tropical savanna is dry, they migrate to where water and food are more abundant. The climates where tropical savannas are found overlap considerably with those of tropical seasonal forests à debate among scientists of by grazing domestic animals and setting fires, humans have increased the extent of tropical savannas in parts of Africa, South America, and Australia.

3.2. TEMPERATE BIOMES a) Temperate deciduous forest Broad-leaved trees that lose their leaves in the autumn and grow a new set in the spring. à decomposition of autumn leaves returns essential nutrients and adds organic matter to forest soils. Moderate summers and cold winters. Precipitation is relatively even throughout the year.

b) Temperate evergreen forest Dominant trees of the temperate evergreen forest keep their leaves throughout the year. Less precipitation and warmer temperatures than the temperate deciduous forest, and summer months are notably drier than winter months. Where heavy winter rains and summer fogs provide ample year-round moisture, evergreen forests are especially lush and diverse, forming temperate rain forests. In both the temperate evergreen forest and temperate rain forest, the dominant trees are towering evergreens.

c) Chaparral Winters are mild and moist and summers are hot and dry à conditions often said to be Mediterranean because they are characteristic of the lands surrounding the Mediterranean Sea.

d) Temperate grassland Grasses interspersed with a diverse array of herbs are the dominant vegetation Climate of this biome is too dry to support forest, woodland, but wet enough to prevent the land from becoming desert. In most temperate grasslands, the climate is decidedly continental: winters are often long and cold, but summers can be quite hot.

3.3. POLAR BIOMES a) Boreal forests Growing season is less than 4 months and winters are long, dry, and bitterly cold à open coniferous forests. Russians call the vast boreal forest of Siberia taiga. Annual precipitation in this biome varies between 500 and 1,500 mm (20 to 59 in./yr), most falling as snow. The cold weather here slows the rate of evaporation, and the relatively flat terrain impedes drainage. As a result, soils usually remain wet or boggy throughout the growing season.

b) Tundra Treeless landscapes dominated by grasses, herbs, and low shrubs. Tundra receives less precipitation than many deserts, but limited evaporation associated with cold weather causes most tundra soils to remain wet through the summer.

3.4. DESERTS All deserts are dry and annual evapotranspiration (the loss of water vapor from soil and leaf surfaces) is much greater than annual precipitation. Range of annual average temperatures among the deserts of the world is quite large: the central portion of the Sahara Desert annual average temperature of 27 °C, the Wright Valley in Antarctica is −20 °C. Many desert plants are succulent, meaning they take up large quantities of water and store it in thick, fleshy stems or leaves. Succulents such as cacti possess abundant spines that protect them from thirsty herbivores.

3.5. HUMAN IMPACTS ON BIODIVERSITY AND BIOGEOGRAPHY Human activities can alter distribution of plants and animals -

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Roads and highways block access for plants and animals Fire suppression halts cyclical succession Removal of animal species • Bison • Wolves Exotic species...